The present invention generally relates to the input stage of signal processing circuits, such as audio amplifiers, and more particularly to input stages of electronic gear which use 3-pin input connectors.
A common problem that plagues professional audio systems using patchable 3-circuit connectors is that there is no universal standard for connecting the pins to be used for the signal, signal return, and chassis common. Different manufacturers, especially manufacturers in different countries such as U.S. and European manufacturers, use different pin connections, and the signal source output configuration from one piece of equipment to another will vary widely and include balanced and unbalanced outputs. Consequently, equipment is often cabled together without matching input and output pin configurations causing unexpected signal inversion, unwanted change of gain, and power frequency injection often referred to as "hum" or "rizz".
The present invention overcomes the above-mentioned problems associated with using unmatched 3-circuit connectors by providing a unique input stage circuit that avoids undesirable changes of gain over a wide range of possible input pin connector configurations. The input circuit of the present invention also eliminates power frequency injection by breaking the ohmic connection between the signal source chassis and the chassis of the piece of equipment using the input circuit of the invention.
More specifically, the invention provides an input stage for an amplifier or the like that will accept a signal source output from equipment of most any manufacturer regardless of connector design conventions. Assuming equal input signal drive levels, the output of the input stage, and thus the signal input to the amplifier, will be the same (except for possible reverse polarities) regardless of which two pins see the signal, and regardless of whether the pins are connected to a balanced or unbalanced output. Moreover, using the input circuit of the invention there will be no input connection that will short the output of the signal source (other than connecting the hot pin directly to the input connector shell.) The input circuit of the invention will accommodate virtually all 3-pin connector standards and permit the user to employ a variety of types of phase reversing adaptors without fear of shorting out the signal source or suffering in any way a change in gain.